Lumber is typically handled in packages. Each package comprises a number of tiers of boards. Some of the tiers may be spaced apart from corresponding adjacent tiers by sticks which extend crosswise to the boards.
A continuous breakdown hoist is used to separate boards from packages of lumber. For example, such a hoist may be used to break a package of lumber into a stream of individual boards that may be fed into a planer in a planer mill. Examples of continuous breakdown hoists are described in:                Jacobsen U.S. Pat. No. 4,640,655        Johnson U.S. Pat. No. 4,838,748 and CA 1318334,        Ritola U.S. Pat. No. 5,249,915 and CA 2089436,        Newnes U.S. Pat. No. 5,879,129 and CA 2230464, and        Hellstrom SE 7501507.3.Other hoists are described in: SE161631; U.S. Pat. No. 5,795,126; CA2121401; SU906358; SE7405427; U.S. Pat. No. 1,676,957 and U.S. Pat. No. 1,549,149.        
The continuous breakdown hoists described above each have a hoist capable of lifting a package of lumber to a discharge location and a secondary hoist, set of accumulator arms, or the like, that can support a package of lumber (or partial package of lumber) while another package of lumber is loaded onto the hoist. This permits a substantially continuous discharge of boards to downstream equipment such as a planer.
The present inventors have determined that the continuous breakdown hoists described above suffer to at least some degree from some or all of the following disadvantages:                The secondary hoist must be located in the feed path that delivers lumber to a planer or other device. There is no way to bypass use of the secondary hoist. Therefore, the secondary hoist must be used for all packages, whether it is truly needed or not.        The secondary hoists are complex mechanically, and subject to mechanical failure. Since they cannot be bypassed, failure of a secondary hoist can cause an entire plant to be shut down at great expense.        Debris such as sticks and snow spilling from lumber packages can interfere with the operation of the continuous breakdown hoist mechanisms.        Some of the designs cannot be retrofitted to an existing conventional tilt hoist.        Designs that provide arms that are located underneath the package, impose further constraints that impair the overall functionality of the machine—i.e. such designs typically provide slots for the arms to extend and index through. Such slots make the machine prone to jamming as a result of debris entering the slots.        In designs that have bottom arms that support a package, the need to lower and retract the bottom arms before raising the main hoist to spill can increase the over-all cycle time.        
To maintain the most profitable operation, it is generally desirable to run a planer mill or other lumber-processing operation at its maximum throughput. The rate at which a planer mill can receive lumber typically depends upon the width and length of the lumber being processed. This rate is often limited by the power available at the cutting heads. Therefore, wider material is typically processed at a reduced linear rate while narrower material can be processed at an increased rate. The capacity of a planer mill may also be limited by a rate at which downstream equipment can receive and handle individual boards. Therefore, the rate at which short wide material can be processed may be constrained by the rate at which the material can be processed by a breakdown hoist. By contrast, the rate at which long wide material can be processed may be constrained by the rate at which that material can be processed by a planer.
The performance required of a breakdown hoist can be reduced by providing surge capacity between the hoist and a planer or other downstream machine. However, increasing the surge capacity takes up space and can cost more to install and operate.
There remains a need for reliable and cost-effective continuous breakdown hoists.